Organic light emitting diodes (OLEDs) are multi-layer optoelectronic devices. The current commercial applications for OLEDs are primarily in display applications These devices are composed of small molecules as the active materials. Manufacture of the small-molecule-based OLEDs relies heavily on vapor deposition processes. The next big application space for OLEDs is general lighting. In order to meet the large volume/low cost requirements for general lighting, low cost manufacturing processes like a roll-to-roll, newspaper-like printing are needed. Such roll-to-roll manufacturing employs solvent-based processes and thus small-molecules are replaced by good film-forming polymers. Processes that use successive solvent-based deposition steps suffer from the potential problem that the solvent used to apply one layer will remove the underlying polymer layer.
The first polymer based OLEDs were simple three-layer devices, limited to a cathode, an anode and an emissive layer, so as to minimize the issue of polymer removal by solvent. Naturally there has been a need to improve device performance thus necessitating the need for additional layers in the OLED.
The requirement for additional layers has led back to the problem of removal of underlying layers; wherein all layers are solvent-deposited. Two basic strategies have been applied to enable solvent-based deposition of multiple polymer layers. One can design a device where subsequent layers are deposited by solvents that do not remove the underlying layer. The prototypical example is that where the hole injection layer composed of poly(styrenesulphonate-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) is applied from water, the subsequent light emitting layer like 9,9-di-substituted polyfluorene, is deposited from solvents like xylenes; PDOT:PSS is not removed by xylenes. A second strategy for preparation of OLEDs with multiple layers deposited by solvents is to render each layer insoluble prior to application of the next layer. The second strategy can be divided into two types. A solvent soluble layer can be combined with a cross-linkable monomer like an acrylate. The desired layer and monomer is then subjected to conditions that cross-link the monomer, typically UV-irradiation. The result of the irradiation of the polymer layer and acrylate monomer is formation of an interpenetrating layer, rendering the solvent-deposited layer insoluble. A more elegant way to render a solvent-deposited layer insoluble is to custom-synthesize the layer of choice so that it contains a cross-linkable site; no added monomers are necessary. After deposition of the solvent-deposited layer, heat or light renders the layer insoluble.
However, there remains a need for methods and compositions that can render solvent-deposited layers insoluble for later deposition steps.